The present invention relates to optical pulse sources, and more particularly to an ultra-short optical pulse source having an output optical pulse width significantly less than ten picoseconds, the source using a gain-switched laser diode with a dispersive delay line. The gain-switched laser diode is selected for minimal satellite temporal pulses as determined by a spectral testing technique.
Short optical pulses are important in various test and measurement applications, including optical sampling and characterization of photodetector impulse response. Temporal resolution in such applications is ultimately limited by the optical pulse duration. For state of the art photodetector characterization and sampling measurements optical pulses with full width at half maximum intensity, dt.sub.FWHM, of less than ten picoseconds are required. Sources of such optical pulses are readily available in laboratory environments, such as mode-locked dye and Nd:YAG lasers, but such sources are generally impractical for use in general test and measurement instrumentation because they are neither robust nor triggerable and have substantial size and power requirements.
For gain-switched diodes these problems are not an issue. Reproducible production of individual optical pulses with dt.sub.FWHM &lt;20 ps has been demonstrated using such lasers with Fabry-Perot optical cavities. However production of such pulses having dt.sub.FWHM significantly less than ten picoseconds using Fabry-Perot diode lasers has not been reported, presumably due to parasitic reactances in both the diode laser and its package as well as the finite storage time of photons in the laser optical cavity. Takada et al have reported in the Journal of Lightwave Technology, Vol. LT-5, No. 10, October 1987 a method for producing diode laser pulses having dt.sub.FWHM of approximately six picoseconds with a gain-switched distributed feedback laser diode (DFB-LD) system. This technique is based on the use of a dispersive delay line to compress the pulsed output of such a laser.
Although the resulting pulses are short enough to be very interesting, assessment of the applicability of such pulses in test and measurement systems requires more detailed knowledge of the pulse temporal profile and pulse-to-pulse reproducibility. As a result of a series of detailed autocorrelation measurements of the dependence of the average optical pulse shape on the length of the fiber used for optical compression, two important shortcomings of the DFB-LD system were identified: pulse tails or satellite temporal pulses as shown in FIG. 1a, and low pulse energy. These two shortcomings are partially interrelated since greater than twenty percent of the pulse energy is located in the satellite temporal pulses. Since these pulse tails are quite long, greater than 40 picoseconds, and contain so much of the pulse energy, they are problematical for impulse excitation or optical sampling measurements, as is shown in FIG. 2 where the ideal pulse response of a photodetector is distorted as indicated by the dotted line.
What is desired is an ultra-short optical pulse source using a gain-switched dynamically single-mode laser diode system that minimizes satellite temporal pulses, and thus increases the pulse energy.